Acoustic wave device

ABSTRACT

A surface acoustic wave (SAW) device including a crystal substrate having two substantially planar surfaces and at least one SAW-to-plate mode coupler positioned on one of the surfaces. The SAW-to-plate mode coupler includes a plurality of parallel, linear surface perturbations. The surface perturbations are adapted to convert a portion of an incident SAW to a bulk acoustic wave (BAW). In addition, the coupler is adapted to convert a portion of an incident BAW (from the crystal bulk region) to a SAW at those surface perturbations. In addition, the planar surfaces of the crystal substrate are adapted to reflect portions of incident BAW&#39;s. The coupler is positioned with respect to the crystal characteristics so that SAW&#39;s and BAW&#39;s resonantly interact at the coupler. In alternative configurations, a second SAW-to-plate mode coupler is positioned on the second surface of the crystal substrate. This second surface coupler also is adapted to convert a portion of an incident SAW to a bulk acoustic wave (BAW) at a plurality of parallel surface perturbations, and to convert a portion of an incident BAW to a SAW at those perturbations. The couplers are positioned in view of the crystal characteristics so that the BAW&#39;s and SAW&#39;s resonantly interact at the couplers.

REFERENCE TO RELATED APPLICATION

The subject matter in this application is related to that in U.S. patentapplication Ser. No. 040,667, filed May 21, 1979, now U.S. Pat. No.4,268,808.

BACKGROUND OF THE DISCLOSURE

The present invention is directed to acoustic wave devices and, moreparticularly, to surface acoustic wave filters.

In the prior art, a number of acoustic wave devices are based on theinteraction between an acoustic wave traveling on the surface of thecrystal and a surface perturbation, such as a shallow groove grating onthat surface. Such devices include filter banks, resonators, andreflective array compressors (RAC's). These prior art devices have allbeen "single surface" devices, that is, the input surface acoustic wave,the surface perturbation, and the output surface acoustic wave are allon the same surface of the crysal.

By way of example, a prior art filter may be constructed on the topsurface of a crystal by placing a shallow groove grating on thatsurface. In response to a broadband surface excitation, a spectrum ofsurface acoustic waves (SAW's) may be generated which travel toward thegrating at an oblique angle. At the grating, the input SAW's are splitinto two components. For a component of the input SAW where theinter-groove spacing (in the propagation direction of the incident SAW)of the grating is equal to the SAW wavelength, a first output SAWcomponent is produced which travels in a direction having its angle ofreflection with respect to the grating grooves equal to the angle ofincidence with respect to those grooves. The other portion of the inputSAW produces a second output SAW component which travels from thegrating along the crystal surface in substantially the same direction asthe incident SAW. The reflection coefficient for the first outputcomponent is proportional to the ratio of the depth of the groove to thewavelength of the incident wave. With this configuration, a sensor whichis aligned to detect the first output SAW component exhibits atransmission pass-band at the frequency associated with the gratingspacing. Similarly, a sensor which is aligned to detect the secondoutput SAW component exhibits a transmission stop-band at the frequencyassociated with the grating spacing. While this prior art configurationdoes provide stop and pass-band filters for a surface acoustic wave, theQ of the band characteristic is relatively low.

Another form of prior art device includes a pair of parallel shallowgroove gratings on a single surface of a crystal, with the elements ofeach grating having a separation equal to one-half the wavelength of thecharacteristic frequency for the resonator. The device provides aresonant peak at the characteristic frequency. However, thisconfiguration is relatively limited with respect to its Qcharacteristic.

The prior art reflective array compressor devices generally include twoshallow groove gratings on a single crystal surface. The elements ofboth gratings have matching monotonic inter-groove separation functions.The two gratings are set at an angle to each other so that a broad bandSAW directed at the first grating travels toward the first grating, andthe various frequency components of that SAW are reflected on thegrooves having a spacing matching the wavelength of the components. Thereflected components travel to the correspondingly spaced grooves of thesecond grating, and are then in turn reflected to an output transducer.By conventional techniques, the two gratings may be adapted to provide aphase response corresponding to a linear chirp, i.e., the phase responseis a quadratic function of frequency. These prior art devices have beenfound to have relatively high loss due to the SAW reflectioncoefficients.

While the above described examples from the prior art are defined tomake use of shallow groove gratings, it is known that other forms ofsurface perturbations may be used, such as films, or field shortingelements for piezoelectric crystals.

It is an object of the present invention to provide improved acousticwave devices.

Another object is to provide a relatively high Q acoustic wave filter.

It is still another object to provide a new and improved acoustic wavereflective array compressor device.

Yet another object is to provide a relatively high finesse acoustic waveresonator.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other objects of this invention, the various featuresthereof, as well as the invention itself, may be more fully understoodfrom the following description, when read together with the accompanyingdrawings in which:

FIGS. 1-5 show exemplary embodiments of the present invention.

SUMMARY OF THE INVENTION

Briefly, according to the present invention, a surface acoustic wavefilter includes a crystal substrate having two substantially planarsurfaces and at least one SAW-to-plate mode coupler positioned on one ofthe surfaces. The SAW-to-plate mode coupler includes a plurality ofparallel, linear surface perturbations. The surface perturbations areadapted to convert a portion of an incident SAW to a bulk acoustic wave(BAW), and to convert a portion of an incident BAW (from the crystalbulk region) to a SAW. In addition, the planar surfaces of the crystalsubstrate are adapted to reflect portions of incident BAW's.

By way of example, the surface perturbations may take the form of ashallow groove grating, an array of film elements, or an array of fieldshorting elements (for crystals that are piezoelectric). Othertechniques may also be used for creating the surface wave/reflectivebulk wave interactions.

With this configuration, conventional surface acoustic wave transducersmay be positioned on the top surface of the crystal on either side ofthe surface perturbations so that the transducers respectively generateand receive acoustic waves travelling in a direction normal to theperturbations. Where the perturbations are periodically spaced, and thecrystal has a predetermined thickness (related to the incident SAWwavelength, the perturbation period, and the characteristic velocity ofBAW in the crystal), the transfer function between the input and outputtransducers is a plurality of relatively high Q stop bands.

In alternative configurations, a second SAW-to-plate mode coupler may bepositioned on the second surface of the crystal substrate. This secondsurface coupler also is adapted to convert a portion of an incident SAWto a bulk acoustic wave (BAW) at a plurality of parallel surfaceperturbations, and to convert a portion of an incident BAW to a SAW atthose perturbations. This second coupler may of course be of the sameform as the coupler on the first surface. In one form, the perturbationsof both first and second surface couplers are periodic and the periodsare equal. In this form, with transducers positioned on oppositesurfaces of the crystal and on opposite sides of the perturbations, thetransfer characteristic between the input and output transducers is aplurality of pass-bands.

In yet another configuration having periodic couplers on both crystalsurfaces, the periods of the first and second surface couplers differ.Here, a portion of an acoustic wave incident on the first surfacecoupler is converted to a BAW which travels to the second coupler. Atthe second surface coupler, a portion of the incident BAW is convertedto a SAW on the second surface. By suitably selecting the crystalsubstrate thickness and the second surface coupler position and periodwith respect to the first surface coupler position and period, eitherthe SAW-to-BAW conversion at the first surface coupler is forwarddirected (i.e. the BAW from that coupler has a velocity component in thesame direction as the input SAW) and the BAW-to-SAW conversion at thesecond surface coupler is reverse directed (i.e. the SAW from thatcoupler has a velocity component in the direction opposite to the BAWincident to that coupler), or the SAW-to-BAW conversion at the firstsurface coupler is reverse directed (i.e. the BAW from that coupler hasa velocity component in the opposite direction to the input SAW) and theBAW-to-SAW conversion at the second surface coupler is forward directed(i.e. the SAW from that coupler has a velocity component in the samedirection as the BAW incident to that coupler). In either case, thetransfer function between an input transducer (on one surface) and anoutput transducer (on the other surface and on the same side as thesurface perturbations as the first coupler) includes a single pass-bandabout at a center frequency having a period between the two periods ofthe surface perturbations. The acoustic wave energy at that centerfrequency travels in a "U-path" from one transducer on one surface tothe coupler on that surface, through the crystal bulk region to theother coupler, and then back to the transducer on the other surface.

In another form of the invention which includes couplers on both crystalsurfaces, the spacing between the perturbations are matching monotonicfunctions for both surface couplers so that a reflective arraycompressor operation is established. The acoustic wave is in the form ofa SAW from the input transducer to the first surface coupler. At therespective perturbations of that coupler, differing phased BAW's aregenerated. These BAW's are converted back to SAW's at correspondingperturbations at the second surface coupler. These SAW's propagate onthe second surface in the opposite direction to the input SAW. Thus, inthis form, a broadband input SAW produces a linear chirp with the inputacoustic wave energy travelling in a "U-path" from a transducer to thecoupler on the first surface, then through the substrate bulk region tothe second coupler, and finally to a transducer on the second surface onthe same side of the surface perturbations in the first coupler.

In yet another form of the invention, a pair of "U-path" filters, havingthe same identical center frequencies, are arranged in a mirror-imageconfiguration on either side of input and output transducers on acrystal substrate. For acoustic waves at the center frequency, eachfilter includes a forward (or reverse) direction coupler on one surfaceand a reverse (or forward) direction coupler. The filters are positionedon the substrate to establish a standing wave at the center frequency,thereby providing a ring path resonator.

DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 shows an embodiment of the present invention in section. Thisform of the invention includes a crystal substrate 10 having twosubstantially planar surfaces 12 and 14. In the present embodiment, thesurfaces 12 and 14 are substantially parallel and separated by adistance T, although in alternative embodiments the surfaces 12 and 14are not necessarily parallel. In the present embodiment, the crystal 10may be YZ Lithium Niobate (LiNbO₃), with a thickness T=1.27 mm. Inalternative embodiments, the substrate might be quartz, for example.

The upper surface 12 of substrate 10 includes a SAW-to-plate modecoupler 18. As shown, the coupler 18 is a shallow groove grating. Theback surface 14 is relatively smooth, and adapted for reflecting bulkacoustic waves. By way of example, the grating may include 200 groovesetched by conventional techniques so that the grooves have a depthd=0.364 micrometers (μm) and a repetitive period P₁ =20.32 μm. Surfaceacoustic wave transducers 20 and 22 are positioned on the surface 12 onopposite sides of the grating. The surface transducers may beconventional in form.

In the present embodiment, the grooves (defined as developed-out regionsin photoresist (Shipley AZ135OJ)) are initially ion beam etched. Thephotoresist on the substrate is then dissolved in acetone. The crystalis etched for 10-15 seconds in a 1:1:1 solution of HF:HNO₃ :H₂ O. Theentire bare surface is then ion etched again to remove about 0.2 μm ofmaterial. Finally, the substrate is again chemically etched for 10-15seconds in the above solution.

With this configuration, a portion of a broadband SAW (produced bytransducer 20) interacts with each groove edge to convert a portion ofthat wave to a bulk acoustic wave (BAW). For the SAW's component havinga wavelength equal to the inter-groove spacing, the elements of thegrating act as a plurality of inphase radiating sources where theresultant BAW is radiated directly downward away from the crystalsurface 12. For the input SAW components having a wavelength differingfrom the intergroove period, the match is no longer perfect for normalincidence, and the resultant BAW propagates away from the grating at anangle for which the phase-matching is satisfied. For SAW components withfrequencies which have wavelengths greater than the grating period, theBAW is "reverse directed", that is the BAW has a propagation directionshifted by more than 90 degrees from the input SAW propagationdirection. For SAW components with wavelengths less than the gratingperiod, the BAW is "forward directed", that is the BAW has a propagationdirection shifted less than 90 degrees from the input SAW direction.

The bulk wave generated at the coupler 18 propagates toward surface 14and bounces off that surface and returns to the coupler 18. Upon thatreturn, a portion of that bulk wave is converted back to a SAW (in a"forward directed" conversion) while a portion of the bulk wave isreflected back to surface 14 of the crystal whereupon the processrepeats. As described more fully below, the thickness of the crystal maybe established with respect to the grating period and the SAWfrequencies, so that resonant SAW-BAW coupling occurs at the coupler 18and the BAW which converts back to a SAW at the surface 12 destructivelyinterferes with the unconverted portion of the input SAW. With thepartial conversion effect, the two surfaces of the crystal provide arepetitive bouncing back and forth of the BAW's and conversions with theSAW's.

With the present embodiment, in response to an input SAW in thefrequency range 160-180 MHz, the resonant BAW-SAW interaction produces atransfer function between transducers 20 and 22 which may becharacterized as a plurality of stop-bands on the order of 10 db deep.By increasing the groove depth to 0.48 um, some of the stop bands becomerelatively sharp with a depth as great as 50-60 db for an input SAW inthe 180-200 MHz range, indicating that substantially all the surfacewave energy in the stop-bands is converted to bulk wave energy.

FIG. 2 shows an alternative configuration which is similar to theconfiguration of FIG. 1, where corresponding elements are denoted byidentical reference designations. The configuration of FIG. 2 alsoincludes a SAW-to-plate mode coupler 30 on surface 14. The coupler 30,as shown, also includes a plurality of linear parallel surfaceperturbations having a uniform periodic separation P₂. In the embodimentof FIG. 2, the grating grooves of couplers 18 and 30 are substantiallymutually parallel although in other embodiments, the two sets of groovesmay not necessarily be parallel. The embodiment of FIG. 2 also showsinput transducer 20 as well as two additional output transducer 34 and36 on surface 14 (on opposite sides of coupler 30). It will beunderstood that the transducers may all be conventional in form, and,depending upon various applications, may or may not actually be in placein all devices in accordance with the invention.

In the preferred form of the device of FIG. 2, the crystal substrate 10is LiNbO₃, has a thickness T=1.27 mm, and the gratings of both couplersare periodic 200 groove gratings with 20.32 um periods, where thegrooves are 0.47 um deep. Moreover, in the illustrated embodiment, thecouplers 18 and 30 are non-overlapping, with the starting point ofcoupler 30 being positioned opposite the ending point of coupler 18.With this configuration, in response to a broadband SAW generated attransducer 20 toward coupler 18, energy transfer from surface 12 tosurface 14 only occurs for a forward directed BAW from coupler 18, andthus only occurs for input SAW frequencies greater than 172 MHz. As aresult, the transfer function between transducers 20 and 34 ischaracterized by a plurality of pass-bands, each having a centerfrequency greater than 172 MHz (i.e. having corresponding wavelengthsless than P₁).

In response to a broadband SAW generated at transducer 22 toward coupler18, energy transfer from surface 12 to surface 14 occurs for a reversedirected BAW from coupler 18, and thus only occurs for input SAWfrequencies less than 172 MHz. Under these conditions, the transferfunction between transducers 22 and 36 is characterized by a pluralityof pass-bands, each having a center frequency less than 172 MHz (i.e.having corresponding wavelengths less than P₁).

In alternative embodiments, the couplers 18 and 30 may overlap with theresult that the transfer functions of diagonally opposite transducerpairs may include pass-bands both above and below the frequencycorresponding to a wavelength P₁.

FIG. 3 shows another configuration which is substantially the same asthe configuration of FIG. 2, except that the periods P₁ and P₂ ofcouplers 18 and 30, respectively, differ. As shown P₁ is greater thanP₂. With this configuration, in response to a broadband SAW generated attransducer 20 toward coupler 18, energy transfer from surface 12 tosurface 14 occurs primarily through the action of a forward directed BAWproduced at coupler 18 and directed toward coupler 30. At coupler 30,this BAW is in part reflected and in part converted to a reversedirected (i.e. toward transducer 36) SAW. A portion of this SAW isreconverted to a BAW as the SAW encounters the subsequent groove ofgrating 30. This BAW propagates to coupler 18 where it is in partreflected, and in part converted to a SAW which resonantly interactswith the existing SAW at coupler 18. The remaining portion of the SAW atcoupler 30 propagates to transducer 36. By appropriately selecting P₁and P₂, there is only a single resonant frequency (having wavelengthbetween P₁ and P₂) which resonantly interacts at both couplers. Aportion of the SAW produced at coupler 30 at the resonant frequencypropagates to transducer 36 so that the transfer function betweentransducers 20 and 36 is characterized by a single pass-band at theresonant frequency, thereby providing a narrow band, U-path filter.

While the present description is directed to the case where the inputSAW is generated at transducer 20, it will be understood that an inputSAW may just as well be applied at transducer 36. A portion of that SAWis converted to a reverse directed BAW which propagates toward coupler18. At coupler 18, this BAW is in part reflected and in part resonantlyconverted to a SAW which propagates toward transducer 20. The reflectedBAW resonantly interacts with the SAW at coupler 30. As a results, theportion of the input SAW at the resonant frequency is coupled totransducer 20. It should be noted that the device of FIG. 3 is a passivereciprocal device in that the input SAW may be applied either by way oftransducer 20 (or transducer 36), with the other transducer 36 (ortransducer 20) being the output transducer. In alternate forms, couplers18 and 30 may not overlap.

In general, the dimensions of the device of FIG. 3 may be determined byconsidering the length of the propagation path of the input SAW throughthe grating of the couplers, in conjunction with the length of thepropagation of the BAW's through the bulk region (from the crystalsurface where the BAW was generated, to the opposite crystal surface,and as reflected back to the originating surface). The grating periods(which control the propagation angle of the BAW's with respect to thecrystal surfaces) and the crystal thickness may be selected to establishorders of plate made acoustic waves which destructively interfere at thecrystal surfaces.

In one form of the invention, the values for P₁ and P₂ may bepredetermined for a known material, such as YZ-LiNbO₃. This material ischaracterized by a Rayleigh wave velocity (C_(r)) equal to 3485 m/sec,bulk share velocity (C_(shear)) equal to 4444 m/sec, bulk compressionalvelocity (C_(compr)) equal to 6883 m/sec. To determine the values P₁ andP₂, there are two conditions which must be solved for a desiredpass-band at frequency, f:

    2f=(C.sub.r /P.sub.1)+(C.sub.r /P.sub.2)                   (1)

and the other condition:

    f.sup.2 /C.sup.2 =[(f/c.sub.r)-(1/P.sub.1)].sup.2 +(n/2T).sup.2 (2)

where d equals the thickness of the crystal, and n corresponds to theorder of the plate made that is excited by using either the bulkcompression or bulk shear velocity for C. By way of example, for aYZLiNbO₃ crystal having thickness T=1.27 mm, a filter having a centerfrequency f=176 MHz, where n is selected to equal 100 and using C=4444m/sec, then P₁ =18.25 um and P₂ =19.64 um. Under these conditions, thereis coupling between the surface waves and a shear type plate mode oforder 100 at the first surface. Furthermore, the second surface gratingwill also couple that plate mode to an oppositely directed surface wave.

Alternatively, the U-path filter of FIG. 3 may be configured byinitially constructing a single surface coupler structure, such as thatdisclosed in conjunction with FIG. 1 including a grating with period P₁.After observing the stop bands generated by that structure resultingfrom coupler 18 and selecting one of those stop bands as the desiredpass-band, the appropriate period, P₂, for the grating of coupler 30 mayreadily be determined using equation (1) above. The resultant structurewith couplers 18 and 30 having periods P₁ and P₂, respectively, thenprovides a single pass-band between transducers 20 and 36 with a centerfrequency at the center of the selected pass-band.

FIG. 4 shows another embodiment which includes two mirror-image portions(enclosed by broken lines A and B), each of which is substantially thesame as the device shown in FIG. 3.

This configuration provides a ring path resonator or oscillator. Each ofportions A and B function as described above in conjunction with FIG. 3.At resonance, an acoustic signal propagates around the device in bothdirections, forming standing waves at the positions of the input andoutput transducers 20 and 36. In alternate embodiments, only one of thetransducers is used, thereby forming a one port resonator. In yet otherembodiments, on or both transducers may be positioned outside thecouplers on the respective surfaces 12 and 14, rather than between thecouplers.

In these configurations, the resultant ring resonator providesrelatively high Q compared with conventional surface acoustic waveresonators due to the avoidance of the propagation loss associated withthe conventional resonators (since in the ring structure, a great dealof the propagation is in the bulk, and bulk wave propagation loss issubstantially lower than surface wave loss).

FIG. 5 shows an alternative band-pass filter configuration which has twocouplers 18 and 30 (in the form of gratings) on a single surface. Thisstructure is relatively easy to fabricate, compared with theconfiguration of FIG. 3 since all gratings and transducers are on asingle surface.

Generally, the filter structuer of FIG. 5 includes a substrate 10,either a crystal or piezoelectric material, having a relatively smoothand parallel upper and lower surfaces 12 and 14. The upper surface 12includes input transducer 20 and output transducer 22. Between thesetransducers, the upper surface includes a first coupler 18 comprising aparallel groove grating having a period P₁. A surface acoustic waveinterrupter 40 is positioned on surface 12 between the coupler 18 andthe transducer 22. As described below, there are a number of possibleforms for this interrupter, with the preferred form being a strip ofrubber cement extending parallel to the grating of coupler 18.Interrupter 40 substantially prevents the propagation of SAW's fromtransducer 20 to transducer 22, while not significantly affecting thepropagation of BAW's within the crystal. The second coupler 30 ispositioned on surface 12 on the other side of transducer 22. In thepresent embodiment, coupler 30 comprises a parallel groove gratinghaving a spatial period P₂. In this configuration, the period P₁ for thecoupler 18 is selected so that portions of an input SAW generated attransducer 20 and directed normal to the grating of coupler 18 isconverted to a bulk acoustic wave (BAW) which has a componentpropagating in the same direction and which reflects back and forthbetween surfaces 12 and 14. The period P₂ of coupler 30 is selected sothat the interaction of this BAW creates a reverse-directed (output) SAWon surface 12, i.e. a SAW moving from the grating of coupler 30 towardstransducer 22. The periods P₁ and P₂ may be selected as described above,so that a single pass-band frequency is established for the transfer offunction between transducers 20 and 22. With this embodiment, the inputSAW may be generated at transducer 22 with the output SAW being receivedat transducer 20.

In practice, for example, a single pass-band filter having a centerfrequency approximately equal to 177.2 MHz may be constructed on YZLiNbO₃ crystal having thickness T=1.27 mm and with gratings of depthd=0.48 um having periods P₁ =20.32 um and P₂ =19.05 um. With theconfiguration sheer type plate modes n=100 may be coupled on both sidesof interrupter 40 to the oppositely directed SAW's of surface 12. Thestop-band frequencies are nearly coincident for the gratings 18 and 30,although stop-band for the grating 30 is slightly lower in frequency.Then, the crystal thickness is trimmed. As the crystal thickness isdecreased, the frequency of stop-band associated with coupler 30increases faster than the frequency associated with the stop-bandassociated with coupler 18. The difference as a function of decreasingcrystal thickness is on the order of 0.014 MHz/um. By way of example,this trimming may be achieved by conventional ion milling until thedesired stop-bands for the two gratings are coincident and a pass-bandis established. Alternatively, the stop-band overlap for the gratingscould be blurred by broadening the stop-band associated with therespective gratings, for example, by chirping one of the grating periodsby a small amount, i.e. slowly varying the period of the grating as afunction of position.

Regarding the interrupter 40 in the structure of FIG. 5, the preferredform of the invention incorporates a SAW absorber such as a thin stripof rubber cement extending parallel to the grating. In alternativeembodiments, an interrupter 40 may be a semi-conducting film, depositedbetween the coupler 18 and the transducer 22 on the surface 12, wherethe film has an appropriate sheet resistance so that it very stronglyabsorbs surface acoustic waves, yet has a relatively small effect onbulk plate modes. In another alternative form, interrupter 40 has asemi-conducting film between the coupler 18 and transducer 22 on thesurface 12, where the film is patterned so that the plate mode"wavelength" along the surface is much longer than the Rayleighwavelength along that surface. This latter form is particularlyeffective if the frequency of the SAW is approximately equal to C_(r)/P, where the C_(r) is the Rayleigh wave velocity and P is the gratingperiod. In this case, the plate mode wavelength along the surface ismuch greater than the grating period, the plate mode is minimallyaffected. In this form, the semi-conductor film minimizes loading. Inyet another form, the interrupter 40 is a wedge (or prism) positionedbetween coupler 18 and transducer 22 on the surface 12. This prism isadapted to deflect, i.e. to shift the angle of propagation) of anincident SAW so that the deflected SAW becomes non-coherent with respectto the bulk wave within the substrate 10.

The invention may be embodied in other specific forms without departingfrom the spirit or essential chracteristics thereof. The presentembodiments are therefore to be considered in all respects asillustrative and not restrictive, the scope of the invention beingindicated by the appended claims rather than by the foregoingdescription, and all changes which come within the meaning and range ofequivalency of the claims are therefore intended to be embraced therein.

We claim:
 1. A surface acoustic wave (SAW) device comprising:A. acrystal substrate having two substantially planar surfaces, said planarsurfaces being substantially parallel and separated by a distance T, B.a first SAW-to-plate mode coupler on a first of said surfaces, saidfirst coupler including:first SAW converting means for converting aportion of an incident SAW to a bulk acoustic wave (BAW), said first SAWconverting means including a plurality of parallel linear surfaceperturbations, and first BAW converting means for converting a portionof an incident BAW to a SAW, said first BAW converting means includingsaid plurality of surface perturbations, C. a first BAW reflector on thefirst of said surfaces including means for reflecting a portion of anincident BAW, D. a second BAW reflector on the second of said surfacesincluding means for reflecting a portion of an incident BAW, whereinsaid second BAW reflector is positioned to reflect a portion of BAW'sincident thereon and propagating from said first SAW converting meansand said first BAW reflector, to said first BAW converting means, andwherein said first BAW reflector is positioned to reflect a portion ofBAW's incident thereon and propagating from said second BAW reflector,to said second BAW reflector, and whereby SAW's propagating on saidfirst surface at said first BAW converting means reasonantly interactwith SAW's produced on said first surface by said first BAW convertingmeans, andfurther comprising a second SAW-to-plate mode coupler on saidsecond surface, said second coupling including: second BAW convertingmeans for converting a portion of an incident BAW to a SAW, said secondBAW converting means including a plurality of parallel linear surfaceperturbations, and second SAW converting means for converting a portionof an incident SAW to a BAW, said second SAW converting means includingsaid plurality of surface perturbations, wherein said second BAWconverting means is positioned to convert to SAW's a portion of BAW'sincident thereon and propagating from said first SAW converting meansand said first BAW reflector, wherein said second SAW converting meansis positioned so that a portion of BAW's from said second SAW convertingmeans is incident on said first BAW reflector, whereby SAW's propagatingon said second surface at said second BAW converting means resonantlyinteract with SAW's produced on said second surface by said second BAWconverting means, and wherein said surface perturbations of said firstsurface coupler are substantially periodic, having period P₁, andwherein said surface perturbations of said second surface coupler aresubstantially periodic, having period P₂, and wherein P₁ differs fromP₂, and wherein said first and second couplers are mutually positionedwhereby at least a portion of a BAW generated by an incident SAW at oneof said couplers is coupled to the other coupler.
 2. A SAW deviceaccording to claim 1 further comprising:an input transducer coupled tosaid first surface on one side of said first and second couplers, saidinput transducer including means for generating a SAW on said firstsurface having a velocity component directed toward said first coupler,and an output transducer coupled to said second surface on said one sideof said first and second couplers, said output transducer includingmeans for receiving a SAW propagating on said second surface and havinga velocity component directed away from said second coupler, andincluding means for converting said received SAW to a signalrepresentative thereof, whereby the transfer function between said inputand output transducers is characterized by a single pass-band having acenter frequency with a corresponding wavelength between P₁ and P₂.
 3. ASAW device according to claim 2 wherein P₁ is greater than thewavelength corresponding to said center frequency and wherein saidsecond coupler includes a portion displaced with respect to said firstcoupler in the direction of propagation of a SAW generated by said firsttransducer.
 4. A SAW device according to claim 2 wherein P₁ is less thanthe wavelength corresponding to said center frequency and wherein saidsecond coupler includes a portion displaced with respect to said firstcoupler in the direction opposite to the direction of propagation of aSAW generated by said first transducer.
 5. A surface acoustic wave (SAW)device comprising:A. a crystal substrate having two substantially planarsurfaces, said planar surfaces being substantially parallel andseparated by a distance T, B. a first SAW-to-plate mode coupler on afirst of said surfaces, said first coupler including:first SAWconverting means for converting a portion of an incident SAW to a bulkacoustic wave (BAW), said first SAW converting means including aplurality of parallel linear surface perturbations, and first BAWconverting means for converting a portion of an incident BAW to a SAW,said first BAW converting means including said plurality of surfaceperturbations, C. a first BAW reflector on the first of said surfacesincluding means for reflecting a portion of an incident BAW, D. a secondBAW reflector on the second of said surfaces including means forreflecting a portion of an incident BAW, wherein said second BAWreflector is positioned to reflect a portion of BAW's incident thereonand propagating from said first SAW converting means and said first BAWreflector, to said first BAW converting means, and wherein said firstBAW reflector is positioned to reflect a portion of BAW's incidentthereon and propagating from said second BAW reflector, to said secondBAW reflector, and whereby SAW's propagating on said first surface atsaid first BAW converting means resonantly interact with SAW's producedon said first surface by said first BAW converting means, andfurthercomprising a second SAW-to-plate mode coupler on said second surface,said second coupler including: second BAW converting means forconverting a portion of an incident BAW to a SAW, said second BAWconverting means including a plurality of parallel linear surfaceperturbations, and second SAW converting means for converting a portionof an incident SAW to a BAW, said second SAW converting means includingsaid plurality of surface perturbations, wherein said second BAWconverting means is positioned to convert to SAW's a portion of BAW'sincident thereon and propagating from said first SAW converting meansand said first BAW reflector, wherein said second SAW converting meansis positioned so that a portion of BAW's from said second SAW convertingmeans is incident on said first BAW reflector, whereby SAW's propagatingon said second surface at said second BAW converting means resonantlyinteract with SAW's produced on said second surface by said second BAWconverting means, and wherein said surface perturbations of said firstsurface coupler are substantially periodic, having period P₁, andwherein said surface perturbations of said second surface coupler aresubstantially periodic, having period P₂, andfurther comprising: a thirdSAW-to-plate mode coupler on said first surface, and a fourthSAW-to-plate mode coupler on said second surface, wherein said thirdcoupler is substantially identical to said first coupler, and saidfourth coupler is substantially identical to said second coupler,wherein further P₁ differs from P₂, and wherein said first and secondcouplers are mutually positioned whereby at least a portion of a BAWgenerated by an incident SAW at one of said first and second couplers iscoupled to the other coupler, and wherein said third and fourth couplersare mutually positioned whereby at least a portion of a BAW generated byan incident SAW at one of said third and fourth couplers is coupled tothe other coupler.
 6. A SAW device according to claim 5 furthercomprising:an input transducer coupled to one of said surfaces betweenthe couplers on that surface, said input transducer including means forgenerating a SAW on surface having a velocity component directed towardone or both of said couplers on that surface.
 7. A SAW device accordingto claim 5 further comprising:an output transducer coupled to one ofsaid surfaces between the couplers on that surface, said outputtransducer including means for receiving a SAW propagating on thatsurface and having a velocity component directed away from one or bothof the couplers on that surface, and including means for converting saidreceived SAW to a signal representative thereof.
 8. A surface acousticwave (SAW) device comprising:A. a crystal substrate having twosubstantially planar surfaces, said planar surfaces being substantiallyparallel and separated by a distance T, B. a first SAW-to-plate modecoupler on a first of said surfaces, said first coupler including:firstSAW converting means for converting a portion of an incident SAW to abulk acoustic wave (BAW), said first SAW converting means including aplurality of parallel linear surface perturbations, and first BAWconverting means for converting a portion of an incident BAW to a SAW,said first BAW converting means including said plurality of surfaceperturbations, C. a first BAW reflector on the first of said surfacesincluding means for reflecting a portion of an incident BAW, D. a secondBAW reflector on the second of said surfaces including means forreflecting a portion of an incident BAW, wherein said second BAWreflector is positioned to reflect a portion of BAW's incident thereonand propagating from said first SAW converting means and said first BAWreflector, to said first BAW converting means, and wherein said firstBAW reflector is positioned to reflect a portion of BAW's incidentthereon and propagating from said second BAW reflector, to said secondBAW reflector, and wherein SAW's propgating on said first surface atsaid first BAW converting means resonantly interact with SAW's producedon said first surface by said first BAW converting means, andfurthercomprising a second SAW-to-plate mode coupler on said first surface,said second coupler including: means for converting a portion of anincident BAW to a SAW at a plurality of parallel linear surfaceperturbations, and means for converting a portion of an incident SAW toa BAW at said plurality of said surface perturbations, wherein saidsurface perturbations of said first surface coupler are substantiallyperiodic, having period P₁, and wherein said surface perturbations ofsaid second surface coupler are substantially periodic, having periodP₂, wherein P₁ differs from P₂, and wherein said first and secondcouplers are mutually positioned whereby at least a portion of a BAWgenerated by an incident SAW at one of said couplers is coupled to theother coupler, and further comprising: means for generating an input SAWon said first surface having a velocity component directed toward one ofsaid couplers, and means for receiving a SAW propagating on said firstsurface and having a velocity component directed away from the other ofsaid couplers, and opposite to said velocity component of said inputSAW, and a SAW interrupter means on said first surface between saidfirst and second couplers, said SAW interrupter means including meansfor substantially interrupting a SAW propagating on said first surfacefrom said generating means and towards said other coupler and saidreceiving means, whereby the transfer function between said generatingand receiving means is characterized by a single passband having acenter frequency with a corresponding SAW wavelength between P₁ and P₂.9. A SAW device according to claim 8 wherein P₁ is greater than thewavelength corresponding to said center frequency.